Nowadays, because LED has advantages of low manufacturing cost, low manufacturing difficulty level, easy and convenient installation and promising development future, LED is used widely in daily life, such as electronic bulletin boards, indicator lights and car taillights, etc.
FIG. 1 illustrates a cross-sectional view of a structure of conventional III-nitride LED. Referring to FIG. 1, there is an epitaxial structure 12 located on a substrate 10. The epitaxial structure 12 of a LED is composed of a plurality of III-V compound semiconductor layers, and the epitaxial structure 12 of a III-nitride LED is composed of a plurality of III-nitride compound semiconductor layers. Then, an ohmic contact metal electrode 14 and an ohmic contact metal electrode 16 are formed on the epitaxial structure 12, so as to form the LED structure, wherein the ohmic contact metal electrode 14 can be p-type or n-type, and the electrical property of the ohmic contact metal electrode 16 is opposite to that of the ohmic contact metal electrode 14.
In the epitaxial process, the p-type nitride semiconductor material of the III-nitride LED is first sufficiently doped, but most of the dopants are passivated by hydrogen. Hence, an additional activation-annealing step has to be performed in the process of manufacturing the III-nitride LED after the LED structure is formed, thereby increasing the doping concentration of the nitride semiconductor material. Generally, the activation-annealing step is performed by a heating method using a furnace or microwave oven, wherein the LED is put in a proper high temperature condition, and then the atoms in the material will be processed the rearrangement of crystal lattice after a certain period of time, so that the contact resistance between the semiconductor and the metal electrode is reduced.